Abstract
The investigations in the American Tropics described in this paper were made during the summers of 1911 and 1912 under the auspices of the Carnegie Institution of Washington: the work was made possible by the invaluable help and kindness of Dr. Alfred G. Mayer, Director of the Department of Marine Biology. The investigations in Temperate waters were made from the Plymouth Laboratory of the Marine Biological Association of the United Kingdom, and my thanks are due to the Council and Director of the Association for giving me every facility for the work.